The present invention relates generally to printing devices which include printheads of the type having firing or heater resistors for effecting the ejection of ink droplets and, more particularly, to a system and method for controlling current density in the heater resistors of such printheads.
Thermal ink jet printer mechanisms which utilize printheads having heater resistors for effecting the ejection of small ink droplets from the printhead are well known. The ejection of a large number of small ink droplets at controlled locations on a printing medium produces a desired printed image.
One problem associated with printheads used in ink jet printer mechanisms is early life failures resulting from electromigration within the heater resistors of the printheads. For example, the resistance of heater resistors made with Tantalum-Aluminum thick film decreases over time as the heater resistors are fired. This change in resistance causes the operating current to increase for fixed voltage drive systems. If the operating current is not controlled, excess current can lead to current density induced electromigration of Al ions to create higher concentration Al ion areas which are lower in resistance and allow even more current. A spiraling resistance decrease can occur that quickly leads to heater resistors burning out in early usage.
Accordingly, it would be advantageous to provide a system and method for controlling the current density through heater resistors of ink jet printheads in order to reduce or prevent the detrimental effects of electromigration.
In one aspect of the invention, a method for limiting electromigration within heater resistors of a printhead of an ink jet printer involves determining a resistance drop off characteristic of the printhead heater resistors and identifying a substantially stable resistance value of the printhead heater resistors based upon the resistance drop off characteristic. The printhead is then run through a resistor bum in operation which involves repeatedly firing a multiplicity of the heater resistors until a resistance value of each heater resistor of the multiplicity drops to the substantially stable resistance value. This technique takes advantage of the fact that the resistance drop off characteristic of the heater resistors tends to stabilize after a certain number of firings. The burn in operation may be performed during the manufacturing stage of a printer mechanism, or may be performed during a printer initialization sequence after delivery of the printer mechanism to a consumer. Further, the burn in operation may be performed in a dry mode where no ink is delivered to the printhead for ejection, but may also be performed in an ink ejection mode.
In another aspect of the present invention, a method of controlling firing of a heater resistor within a printhead of an ink jet printer involves establishing a desired current density or maximum acceptable current density for the heater resistor. A resistance of the heater resistor is thereafter monitored during printing operations. Based at least in part upon the monitored resistance, a firing voltage across the heater resistor is adjusted in order to maintain the desired current density through the heater resistor or in order to hold current density through the heater resistor at or below the maximum acceptable current density. Thus, the method accounts for changes in resistance of the heater resistor over the course of operation of the printer. Various techniques can be used to track the resistance of the heater resistor.
A further aspect of the present invention provides a printer mechanism including a printhead having a plurality of heater resistors. A printhead driver circuit is operatively connected to the printhead for selectively energizing the heater resistors. A controller is connected with the printhead driver circuit for effecting operation of the driver circuit. The controller is operable to determine a resistance of the printhead heater resistors and to adjust a firing voltage across the heater resistors based at least in part upon the determined resistance value. The printer can therefore be utilized to maintain a desired current density through the heater resistors or to hold current density through the heater resistors at or below a maximum acceptable current density.